The tread of conventional vehicle tires typically comprises a plurality of load bearing tread block elements separated by a network of grooves. The tread block elements are arranged in a pattern around the circumference of the tire. As the tire rolls over a road surface, the tread block elements move into and out of engagement with the road surface causing disturbances in the surrounding air molecules that generate sounds. For any given tread pattern a certain amount of acoustic energy is produced. The distribution of the acoustic energy over the audio frequency spectrum has been found to be controlled at least in part by the geometry of the tread block elements and grooves. Where tread block elements of constant pitch length have been used to completely surround a tire, the sound generated is dominated by a single frequency and its harmonics, i.e., a majority of the sound generated is concentrated into a small frequency range of the audio frequency spectrum. This tonal concentration may be highly disturbing to the occupants of the vehicle on which the tire is mounted.
Much effort has been directed to the reduction of objectionable tire noise, with emphasis on the spreading of the energy produced by the tire tread over the audio frequency spectrum thereby to reduce objectionable tonal concentration. A technique known as "pitching" accomplishes this by varying the length, i.e., the pitch, of the design cycle of the tread of the tire around its circumference, with the result being a variable pitch tire tread pattern. The design cycle is a representative portion of the tread which typically comprises in the circumferential direction at least one tread block element and a generally transversely extending groove. For conditions of practical manufacture of the molds for the tires, the circumference of the tire is composed of an integer number of design cycles. The lengths of the design cycles may be varied in a random or preselected algorithmic manner to distribute the audio noise throughout the frequency spectrum.
In order to avoid the high cost of building and testing tires with different tread patterns, computer simulations have been used to perform tread pattern analysis. A common technique in the tire industry regarding pitch sequence analysis is to view the Fourier spectrum of the entire pitch sequence. See, for example, SAE paper 951352 entitled "Tire Tread Pattern Noise Reduction Through the Application of Pitch Sequencing." Such technique provides meaningful information about the acoustical output of a tire pitch sequence which aids in the task of reducing objectionable tire noise. This enables various pitch sequences to be analyzed in order to identify an optimal pitch sequence that can then be applied to a test tire for actual physical testing.